1. Field of the Invention
The present invention relates to an apparatus in the form of a die assembly for building a laminated core and more particularly to improvement relating to a die assembly in which skewing and turn laminating are simultaneously carried out for automatically laminating a number of core sheets to build a laminated core for an electric motor.
2. Description of the Prior Art
Generally, a laminated core for an electric motor is produced using a hoop of magnetic steel plate by way of the steps of punching core sheets in one or more lines to a predetermined configuration and then successively laminating them one above another in the integrated structure. As is well know, a strip of steel plate does not have an uniform thickness as seen in the transverse direction due to bending of the rolls during the rolling operation. FIG. 1 is an exaggerated cross-sectional view of steel plate sectioned in the transverse direction. As is apparent from the drawing, it has a flattened trapezoidal cross-sectional configuration. For the same reason a hoop of steel plate which is prepared by slitting a strip of steel plate also has a trapezoidal cross-sectional configuration. Particularly, in the case of a hoop obtained from both the side parts PLa and PLb of a strip has remarkable fluctuation in thickness on both sides of the hoop. This tendency of increased fluctuation in thickness becomes more and more severe as the hoop has an increased width. When a laminated core is built by using core sheets having such trapezoidal cross-sectional configurations, there results a core with a trapezoidal cross-sectional configuration that is called banana shape (see FIG. 2).
To obviate this problem there has been proposed a method of laminating core sheets in such a manner as to cancel fluctuation in thickness (as disclosed in Japanese Patent Application No. 32486/1980). Specifically, in the proposed method oral punched core sheets are laminated one above another while they are turned by an angle of 180 degrees after completion of the punching operation of one core sheet in the blanking die which is arranged in the station where a hoop of steel plate is successively punched.
However, the prior method as mentioned above has drawback that inclined extension of slots in the laminated core that is called skew fails to be generated.
Further, as other typical prior methods there were proposed a method of mechanically turning the blanking die by a small angle by utilizing every stroke of the press machine (Japanese Patent Application No. 181232/1983) or turning the blanking die by a small angle by applying the number of pulses equal to required skew to a stepping motor so that skewing is achieved for the laminated core (Japanese Utility Model Laid-Open No. 10372/1980). However, the first-mentioned prior method still has the drawback that turn lamination fails to be carried out, whereas the last-mentioned prior method has drawbacks that it takes a long time to carry out feedback control to assure skew angle at high accuracy. Moreover, punching cannot be achieved at higher speed, although the prior method makes it possible to turn the blanking die by the rotation angle required for generating both turn lamination angle and skew angle.
For instance, in a case where skew angle is generated by means of stepping motor, that is, a case where the blanking die is turned, for instance, by an angle of 0.3 degree by one stroke of operation of the press machine, it is assumed that a stepping motor generating 400 pulses at every rotation by one revolution is employed at a speed reduction ratio of 1/90.
In this case the stepping motor has the following rotation angle. EQU 0.3 degree.times.90=27 degrees
When it is assumed that the required number of pulses is (27/360).times.400=30, resolving capability amounts to 0.3 degree/30=0.01 degree in terms of angle and the average number of pulses is 500 pulses/second, it results that time required for rotation by one revolution amounts to 35/500=0.06 second/skew.
Further, when it is assumed that turn lamination angle is determined (for instance, 90 degrees) and the blanking die is turned for both turn laminating and skewing (by an angle of 0.3 degree), rotation time amounts to the following value. EQU 9030/500=18.06 sec./(turn laminating+skewing)
Thus, punching fails to be effected at higher speed. Even when the average number of pulses is increased by four times to reach 2000 pulses/second while maintaining a speed reduction ratio of 1/9, rotation time is reduced only to the following level. EQU 903/2000=0.452 second/(turn laminating+skewing)
Thus, punching is still carried out at lower speed and resolving capability is still held at a lower level of 0.1 degree.
On the contrary, there was recently proposed an apparatus for mechanically indexing turn lamination angle and electrically indexing rotation angle for generating skew angle (see Japanese Patent Application No. 183734/1984). The proposed prior apparatus is constructed such that the output shaft is rotated by a certain angle during rotation of the input shaft of the indexing mechanism to which rotation of the main spindle on press machine is transmitted, said certain angle (that is called cam indexing angle) being equal to turn lamination angle, and rotation of the output shaft is then transmitted to the differential gear mechanism, while output from the servomotor (or stepping motor) is transmitted to another input shaft of the differential gear mechanism via the speed reduction mechanism so that rotation is achieved by an angle equal to rotation angle required for generating skew angle. Owing to the arrangement that indexing of turn lamination angle is carried out in the mechanical way the servomotor is caused to operate with reduced load and moreover owing to the arrangement that the speed reduction mechanism is located behind the servomotor skew angle can be obtained at higher accuracy.
However, it is found that the prior apparatus can be adapted only to the maximum speed level of 300 SPM. Other drawbacks of the apparatus are that it is complicated in electrical system and moreover mechanical components constituting the apparatus are also complicated in structure, resulting in increased manufacturing cost.